Drawing instruments or cutting tools have been proposed with various mechanical structures for making the drawing or tracing of an ellipse easier and more accurate than the typical manual method of tracing with curved templates. Particularly, it is desireable to draw an ellipse in one complete rotation, rather than compounding partial curves, in order to draw an ellipse with a smooth, uniform line that returns accurately to its starting point.
As one example, shown in U.S. Pat. No. 4,182,043 to Nemoto, a driving member is slidably supported on a pair of rails mounted on a pair of spaced apart stanchions. The rails extend along a longitudinal axis of the ellipse to be drawn. The driving member is slidably coupled in a slot in one end of a first arm for driving the arm in rotation. The other end of the first arm is connected by a pin to one end of a second arm, and a drawing stylus is fixed to the other end of the second arm. A pair of supporting arms, having their ends coupled to spur gears movable about respective bevel gears, extend symmetrically from the stanchions and are rotatably coupled to the pin in order to constrain the movement of the pin along a tranverse axis of the ellipse. An ellipse is drawn by rotating the driving member one complete rotation. The position of the coupling of the driving member in the slot can be adjusted to set the diameter of the ellipse along its transverse axis, and the position of the stylus on the second arm can be adjusted to set the diameter along the longitudinal axis.
However, this type of ellipsograph structure has several significant problems. The spur and bevel gear construction for movably supporting the ends of the supporting arms is quite complicated to manufacture, assemble, or disassemble. Further, the range of ellipses that can be drawn is limited by the fixed spacing of the two end stanchions and the chosen dimensions of the supporting arms, spur gears, and bevel gears. Moreover, the center of the ellipse is constrained at the midpoint of the two end stanchions, making it difficult to use the ellipsograph in circumstances where the two end stanchions cannot be positioned over the area where the ellipse is to be drawn. Another ellipsograph structure, as disclosed in U.S. Pat. No. 3,237,309 to Vogel, uses an epicyclic gear and a sliding transverse tube to support the second (drawing) arm, but has similar limitations as the Nemoto structure.
In U.S. Pat. No. 2,156,417 to Witherspoon, an ellipsograph structure has the pin axis between the first and second arm contrained along the transverse axis by a straight line motion mechanism which is mounted from only one end stanchion. This overcomes the limitation on the range of ellipses that can be drawn, and also allows drawing over desired areas without constraint from any required placement of the other end stanchion. However, the Witherspoon straight line motion mechanism is very complex in construction, having a fixed pivot point, a free swing point, and scissors-type pairs of arms movably biased by a tension spring mounted from the one end stanchion. This complicated structure would be difficult to manufacture and assemble, and to disassemble for replacement with a motion mechansim of different dimensions.